Relationship between remaining antioxidant content and radiation-thermal degradation in crosslinked polyethylene

To clarify the deterioration mechanism of cables installed in nuclear power plants, three kinds of XLPE sheets with various antioxidant contents were aged by 100 Gy/h γ-ray irradiation at room temperature, 60, and 100°C. The remaining antioxidant content estimated from an oxidation induction time of a differential calorimetric thermogram as well as the elongation at break decreases faster as the initial antioxidant content decreases. On the other hand, an evaluation parameter called oxidation degree, which is the absorbance ratio of the carbonyl group to the methylene group obtained by infrared spectroscopy, increases with aging time. The oxidation degree increases with an increase in the elongation at break, and the relationship of these two parameters can be plotted around a sigmoid curve. Furthermore, the elongation at break and oxidation degree show reasonably stable transitions until the antioxidant content decreases to a certain value. After such a period, the elongation at break starts to change drastically and becomes sufficiently low value. It is clarified from these experimental results that there exists a certain antioxidant content that effectively inhibits oxidation.